1. Field of the Invention
The present invention relates generally to optical security devices, more particularly to optical security devices and identification for articles, and particularly, documents and financial instruments.
2. Related Art
Surface relief holograms, also known as embossed holograms, have been utilized to authenticate financial instruments and high value products for many years. The security provided by embossed holograms is limited by their low surface relief, typically 0.25 micron, which makes them susceptible to counterfeiting: stripping the hologram from the substrate exposes the complete holographic microstructure which can be easily used to create counterfeit tooling.
Stripping the hologram from the substrate is effective regardless of the optical complexity of the hologram. Limited protection from physical replication has been achieved by patterned metallization and the use of frangible materials, but these approaches do not prevent holographic copying methods. Although photopolymer ‘volumetric’ holograms are not susceptible to physical counterfeiting methods, they can be copied holographically. Moreover, photopolymer holograms also suffer from high production and application costs and small worldwide production capacity.
Therefore, there is a need for compositions and methods for authenticating or identifying articles including identifying reproductions.